The present invention is directed to determining the geographic location of a mobile radio communications transmitter that is part of a wireless cellular communications system so as to facilitate emergency service responses, roadside assistance, traffic monitoring, or other services that can apply or be supported by the location information.
Cellular-telephone systems now provide ready access to wireless telephone communications. Cellular telephones typically operate in an analog system of frequency division multiple access (FDMA). Digital technologies, including time division multiple access (TDMA) or code division multiple access (CDMA), offer greater capacity and should give more individuals simultaneous access to cellular telephone services. In addition, "cellular-like" communications systems, such as a personal communication system (PCS), may further increase the number of individuals with access to a wireless communication network.
A cellular-telephone or cellular-like communication system is a system with a network of fixed base stations serving local areas (i.e., "cells") providing an integrated communication service to a plurality of mobile transmitter/receiver ("transceiver") units, e.g., cellular telephones. Such a communications network attempts to communicate with each transceiver from the base station which provides the optimal communication. The optimal base station is usually, but not necessarily, the one nearest the mobile transceiver. To provide the optimal communications support, the network need not locate the geographic position of the mobile transceiver more accurately than needed to determine which base station to use.
The inability of existing communication networks for cellular-telephone or cellular-like communication systems to accurately determine the location of a mobile transmitter is a major disadvantage in an emergency. For example, public safety officials in Los Angeles estimate that, today, a quarter of all who call the emergency number ("9-1-1") from a cellular telephone do not know where they are when they call. The time spent in finding their location delays emergency assistance units, for example, police or ambulance services, in providing assistance. Other studies indicate that in excess of sixty percent of traffic fatalities in the United States occur on rural roadways. Delays caused by uncertainty in location exacerbate the inherently longer response times for providing emergency services in rural areas.
The problem of locating the position of a mobile radio transceiver has been solved in many ways for many years but in systems other than that of a cellular-telephone or cellular-like communication system. No simple, low-cost solution has been found that is practical when applied to the wide-scale monitoring of mobile telephones. One practical difficulty in implementing any type of localization for mobile radio transceivers is the cost of the modifications either to the transceiver or to the communications network (infrastructure) that are needed to determine the location of the mobile transceiver. Any given transceiver would rarely, if ever, be used in placing a request for emergency or roadside assistance. Thus, the suppliers of transceivers and the operators of communications networks have little economic incentive to increase the complexity (and cost) of the transceivers or to install an extensive and expensive infrastructure to support such rarely used services absent government mandate. However unprofitable in the short term, the value of emergency assistance and roadside assistance services have unquestionable value for providing and enhancing personal and public safety. Ameliorating the increasing incidence of violence and the related, growing concern for personal security with a mobile communications system is a worthy policy goal with the potential for realizing enormous benefit to subscribers, network operators, and the general public alike. However, realizing the objective, even one so important and valuable, requires a practical, inexpensive infrastructure for uniquely identifying people requesting or reporting the need for assistance, communicating with them, and providing their locations to a responding assistant.
Techniques exist for accurately determining one's position in applications other than that of providing emergency or roadside assistance. For example, the satellite-based Global Positioning System (GPS) allows determination of the location of the point of GPS signal reception with a special-purpose receiver for the wireless GPS signals that are broadcast from the satellites. However, obtaining the position of a communications transceiver by using GPS requires the mobile transceiver to include a GPS receiver. GPS receivers are expensive. Even if their cost were to be reduced through mass production, GPS receivers would still have to be integrated with all existing and future mobile transceivers. The cost associated with this solution seems to be prohibitive in view of the infrequency of use of the service and especially in terms of the large number of mobile transceivers already in use both in the U.S. and abroad for which the localization capability is desired.
Techniques also exist for locating the position of mobile communications transceivers by passively monitoring their radio emissions. However, with the simplest of approaches, radio localization does not take into consideration the distortions in apparent location caused by multipath interference (multipathing). Multipathing involves radio signals bouncing off of objects such as vehicles, buildings, hillsides, etc. Without consideration of these effects, the apparent position of the transceiver will be distorted. Multipath propagation is common for short-wavelength, radio communications since relatively smaller objects can reflect substantial amounts of the transmitted signals, and it is especially common in cities with buildings reflecting the signals. The potential, multipath-induced distortions in the apparent position of the mobile transceiver is therefore a problem that must be addressed in passively localizing radio emitters to support applications such as the provision of emergency or roadside assistance.
Multipath propagation conditions need not impede locating a transceiver when signal analysis and source localization procedures are used to ameliorate potential distortions in apparent position. For example, U.S. Pat. No. 4,728,959 to Maloney et al. demonstrates how direction finding procedures, by which the direction angle of the arrival (DOA or AOA) of a signal can be measured, can be applied with two or more receiving base stations. Using the passive monitoring of communication signals that is described in this patent to determine location is an excellent application in that it allows for locating a mobile transceiver anywhere in a service area of a network having at least two receiving stations of known location. The direction finding approach is simple and accurate, but requires a directional antenna at each receiving site. With similar attention to multipath effects, the requirement for directional antennas can be mitigated by the use of time-difference-of-arrival (TDOA) measurements, which can be obtained with omni-directional antennas and with accurate time-base maintenance facilities. Localization with TDOA measurements requires reception at three or more sites, since each pair of sites only enables one TDOA measurement, and each TDOA measurement only specifies a hyperbola (in two dimensions) along which the transmitter can be. Even in a multipath-affected environment, TDOA measurements with at least a triplet of receiving sites can be analyzed to obtain transmitter locations. However, the necessity of requiring joint reception of a common signal at three or more sites with time-maintenance facilities can increase the complexity and cost of the TDOA approach beyond what some cellular telephone or PCS companies are currently or may be willing to accept.
Often, in addition to timing and directional data that can be derived from received signal characteristics, other information is available or can be obtained that relates to the position of a mobile radio transceiver. For example, in a system designed to provide emergency roadside assistance, we may presume that the person requesting assistance is in a vehicle that is on or near a road. Such a presumption may be verified, for example, by asking the person placing the call if he or she is on a road. This type of additional geographic or topological information, called here "collateral information," is of a type that is normally available to a dispatcher. Combining collateral information with the timing information from two (rather than three or more) base stations can define the location of a mobile radio transceiver well enough to make it possible to dispatch emergency and roadside assistance services. The derivation of the position of the transceiver solely from observed characteristics of its radio emissions received at only two sites is adequate, and the need for additional base stations to derive location thus becomes redundant. However, no proposal to date has sought to use such collateral information to make redundant the need for additional base stations.
Monitoring mobile transceivers that are located on vehicles has advantages other than providing support for responses to requests for assistance. One such advantage is enabling the cost effective monitoring of traffic flow. Unplanned traffic incidents ("traffic jams") clog the highways with a resulting deleterious effects on safety, environment, and economy. The volume of message traffic in a major metropolitan area is a type of collateral information, and it can be combined with observed location- and speed-related information and topographic information (e.g., road maps), to indicate which roads are passable and which are congested. However, traffic flow information, emergency services, and roadside assistance, are not the primary reason for establishing a communication system and thus are not provided currently by communications systems. The cost of adding equipment to the communications infrastructure to provide traffic flow information seems justifiable to communications companies only if it can be done using the most modest of infrastructure enhancements.
Today, techniques exist that provide partial and complex solutions to the problem of providing geographical locations with sufficient accuracy to aid emergency and roadside assistance personnel. However, such systems rely on observed information derived from two or more directional receptions, and three or more time-tagged receptions of radio emissions, or on navigation information from devices extraneous to the communications transceiver. No system seeks to obtain location information from the combination of observed timing information, derived from only a pair of communications radio receptions, with collateral information obtained, for example, from street maps. Therefore, it is an object of the present invention to provide a simple and effective way to identify and locate a mobile radio transceiver in any wireless communication system, including those already existing or that are contemplated, such as those for personal communication systems (PCSs), cellular telephones, specialized mobile radios (SMRs), and personal digital assistants (PDAs). It is an object of the present invention to provide an automatic location identification (ALI) and an automatic "number" identification (ANI) that facilitates national and international rural and urban emergency notification and personal security, and roadway monitoring by combining observed information derived from received radio emissions with collateral information derived from street maps, user descriptions, and other information sources.
It is also an objective of the present invention to include: providing a system in which location and identification are provided cheaply as adjuncts to communications for national and international wireless enhanced 9-1-1 (E9-1-1) emergency and routine roadside assistance notification; estimating roadway speed and providing general transportation information such as traffic congestion and flow characterization; providing such capability in a system which is both relatively easy to deploy and inexpensive to construct; providing a system which has a transportable configuration and, therefore, can be used to temporarily monitor localized regions such as road construction areas or the localities of special events such as sporting competitions, conventions, or concerts; providing a combination of processes and attributes to form an inexpensive yet robust system for localization and identification as an adjunct to a communications system.
The present invention provides an apparatus for locating a mobile radio communications transceiver in a wireless communications system that comprises two sensor stations of known location, each sensor station having a receiving antenna to receive a radio signal from the mobile transceiver, a clock mechanism such as a GPS-based receiver or rubidium clock mechanism to maintain a synchronous inter-site time standard, a signal characterization processing unit for determining the time of arrival (TOA) of a specific component of the radio signal transmitted from the mobile radio transceiver to the sensor station, a source of collateral information about other signal characteristics or the environment of operation of the mobile transceiver, a multidimensional parametric correlation processing unit for determining a probable position of the mobile transceiver from TDOA and associated collateral information, and an output indicative of the probable position of the mobile transceiver.
The present invention provides for locating a mobile radio transceiver in a cellular-telephone or cellular-like communications system using a simplified system for passively monitoring signals emitted by the mobile transceiver. In this invention, the processing at two receiving base stations of known location with inter-site synchronization determines a TDOA locus (e.g., two dimensional hyperbola) for the mobile transceiver location. This TDOA locus is then combined with collateral information to determine the likely location of the transceiver. The present invention has particular applicability to roadway transportation in that it facilitates emergency (9-1-1) services and roadside assistance, and it permits the passive monitoring of traffic flow. The collateral information includes location information derived from other than radio location methods. Such information can include the topological information of a map of the roadways in the area of the base station, or other information such as derived speed, if any, of the transceiver, or information obtained from communications from the caller in person or from equipment at the caller's location.
The present invention does not require determining position by combining two or more equivalent hyperbola from three or more base stations; two base stations can be enough. This capability may have particular usefulness in a CDMA communications network in which increased capacity is obtained through dynamic power control so that fewer base stations are likely to receive a transceiver's emissions. Nevertheless, there is nothing in the present invention that precludes using more than two base stations to further confirm the accuracy of a location or to permit locating mobile radio transceivers for which collateral information is not otherwise available. The ability to determine location from two sites using TOA/TDOA methodology has particular benefit for providing emergency assistance in that two-site reception is applicable in more environments, requires less infrastructure, and offers greatly reduced cost. The present invention is particularly useful for monitoring traffic in rural areas, where there are fewer roads with cells often located close to roads, and whose signal capture at three or more sites is highly unlikely. Thus, in rural areas, collateral information in the form of roadway topology better indicates the location of the mobile transceiver along the observed TDOA locus (hyperbola). The present invention also provides a method and apparatus for locating a mobile radio transceiver in a wireless communications system, comprising two or more sensor stations of known location, a method and means for determining a TDOA locus for the mobile radio communications transceiver, and a method and means for combining collateral information with the TDOA locus to determine the location of the mobile radio transceiver.
The present invention has the advantage of being able to determine the location of a mobile radio transceiver without requiring an embedded or integrated special-purpose device, such as a GPS receiver, with the mobile transceiver. Indeed, the present invention enables the localization of all existing cellular telephones. The cost of deploying a location system of the present invention is low. This low start up cost means that the system can be deployed faster so that consumers can realize the benefits sooner and at less expense.